Field of Invention
The present invention relates to a rare earth permanent magnet field, and more particularly to a high-performance NdFeB permanent magnet produced with NdFeB scraps and a production method thereof.
Description of Related Arts
Because of the excellent magnetism, the rare earth permanent magnet material is more and more widely applied in medical magnetic resonance imaging, computer hard disk driver, sound system, and mobile phone. With the energy-saving and low-carbon economy requirements, the NdFeB rare earth permanent magnet material is further applied in fields of auto parts, household appliance, energy-saving control motor, hybrid electric vehicle, and wind power generation.
In 1983, Japanese patent publications JP1,622,492 and JP2,137,496 firstly disclosed a NdFeB rare earth permanent magnet material, and characteristics, components and a production method thereof. American patent publications U.S. Pat. Nos. 6,461,565, 6,491,765, 6,537,385, 6,527,874 and 5,645,651 also disclosed the production method of the NdFeB rare earth permanent magnet.
Conventionally, in order to produce the high-performance rare earth permanent magnet material, a vacuum melting rapid-solidifying method is generally adopted to prepare the rare earth permanent magnet alloy. According to the conventional vacuum melting rapid-solidifying process, the rapid-solidifying alloy raw materials, such as the pure iron, the ferro-iron, the rare earth raw materials and other additive metals, are added into the crucible at one time for melting, in such a manner that the relatively precious raw materials such as the rare earth may volatilize under a high temperature during a melting process. Moreover, under an atmosphere environment, the addition of the raw materials into the crucible leads to the oxidized rare earth material and an increase of the generated slags during melting. The above factors affect the use ratio of the precious metal material, and lead to certain wastes. The vacuum melting rapid-solidifying furnace, produced by Ulvac Japan Ltd., adopts a design of secondary loading, for filling the loading space caused by the melted raw materials in the crucible during the melting process, and increasing a loading amount. However, the vacuum melting rapid-solidifying furnace is failed to solve the problems of the loss of the precious alloy raw materials under the high temperature and the seriously generated slags caused by melting the rare earth raw materials.
During the producing process of the NdFeB rare earth permanent magnet, the NdFeB raw materials are generally melted into the alloys, then the NdFeB alloys are sintered into the NdFeB block through the powder metallurgy method, and finally the NdFeB block is machined into the parts having different shapes. Because NdFeB is hard and crisp, during machining, a large number of leftover materials are generated. Moreover, as time goes by, because of having the fault or reaching the end of the service life, some mechanical devices produced with the NdFeB rare earth permanent magnet are out of use, and a large number of the scrapped NdFeB permanent magnet are able to be recycled. Because the rare earth permanent magnet material has a relatively high cost, people in the industry are always trying to research and develop a method for recycling the rare earth permanent magnet scraps, such as the rare earth permanent magnet defective products, the leftover materials, and the scrapped NdFeB permanent magnet, so as to decrease a raw material cost of the rare earth permanent magnet material and save the available natural resource. Because the above rare earth permanent magnet scraps have a relatively high oxidation degree, if the scraps as the melting raw materials are remelted and recycled, a large number of the slags will generated during melting. The above problem limits the scrap remelting process and the wide application thereof. Thus, the related Japanese enterprises generally recycle the rare earth permanent magnet scraps without adopting the remelting process. For example, Chinese patent application, ZL99800997.0, and American patent publication, U.S. Pat. No. 6,149,861, disclosed a method for recycling the sintered NdFeB scraps. According to the method, the scraps are processed with crushing, pickling and drying, and a product is obtained; then the product is processed with a calcium reduction treatment, and then the recyclable raw material alloy powders are obtained; through adding other alloy powders into the recyclable raw material alloy powders, a composition thereof is adjusted; and finally the sintered NdFeB permanent magnet material is produced. Chinese patent application, ZL02800504.X, and American patent publication, U.S. Pat. No. 7,056,393, disclosed a method which utilizes the sintered NdFeB defective products. According to the method, the sintered NdFeB defective products are processed with coarse grinding through a hydrogen decrepitation process, and the fine powders are formed; the fine powders produced by the defective products are mixed with the fine powders produced by the normal raw materials, and finally the sintered NdFeB permanent magnet is produced. The above methods, which utilize the scraps without adopting the remelting process, not only have the relatively complex process, but also have to prepare the alloy powders having the different compositions for adjusting the composition and improving the sintering performance thereof, which brings troubles to the producing process. More importantly, according to the above scarp utilization methods, because the scraps are not remelted, the powders produced by the scraps have a relatively high content of oxygen and other impurities, so that the magnetic performance of the obtained rare earth permanent magnet material is greatly affected.
With the application of the NdFeB rare earth permanent magnet in wind power generation, automobiles, servo motors, energy-saving motors and electronic devices, the consumption of the heavy rare earth element, Dy, becomes more and more. Dy is a scarce heavy rare earth resource and few in the world, and now only produced from the ionic mineral in south China. A decrease of the consumption of Dy is important for protecting the scarce resource and decreasing the cost of the NdFeB rare earth permanent magnet.
In order to increase the magnetic performance of the NdFeB rare earth permanent magnet material and meanwhile decrease the consumption of heavy rare earth materials such as Dy and Tb, Japanese enterprises have made a lot of researches. Japanese Shin-Etsu Chemical Co., Ltd. in Chinese patent publications, CN100520992C, CN100565719C, and CN101404195B, disclosed a high-performance R—Fe—B permanent magnet containing Dy, Tb, F and O. The average concentrations of F, Dy and Tb gradually increase from a center of the magnet to a surface of the magnet, and the distribution trends thereof are showed in FIG. 1. Moreover, rare earth oxyfluorides exist in the grain boundary of the grain boundary area which is from the surface of the magnet to an interior of the magnet at a certain depth. The permanent magnet is prepared through steps of: sintering the NdFeB magnet; adding oxides, fluorides or oxyfluoride powders containing Dy and Tb on the surface of the magnet; processing the magnet with a thermal treatment at a temperature lower than a sintering temperature in vacuo or an inert atmosphere; and absorbing Dy and Tb in the powders into the magnet. Through the above method, the coercive force of the sintered NdFeB permanent magnet is increased to a certain extent. However, according to the above method, the thermal treatment, which enables Dy and Tb to penetrate into the magnet, proceeds after sintering, causing the magnet becoming more crisp and harder, which brings troubles to subsequent machining and processing, leads to the easily broken edges and corners of the products during the transport process, and increases the rejection rate of the products.